Protein-mediated water transport is a fundamental physiological process in all organisms. It is carried out by integral membrane proteins, aquaporins, that function as water channels. Aquaporins serve as passive, diffusion-limited channels to dissipate osmotic gradients that form across cell membranes. Currently, six different human aquaporins have been discovered, each with a different distribution in bodily organs, tissues and cells. This heterogeneous distribution predicts significant roles for human aquaporins in both normal physiology and disease. Mutations in one aquaporin, aquaporin-2, are responsible for nephrogenic diabetes insipidus. Others are implicated in the maintenance of water homeostasis in erythrocytes, kidney, lung, brain and salivary gland. The aquaporins are likely targets for the future development of therapeutic agents directed to prevention or control of edema and fluid balance. The molecular basis of protein-mediated water transport will be elucidated by crystal structure determination of human aquaporins. Purification of aquaporin- 1 (AQP- I), the archetypal aquaporin, from erythrocytes yields quantities of protein suitable for crystallization. Crystals that diffract to 3.5A have been obtained, and a complete 4A dataset has been collected. Mercury/AQP-1 co-crystals have been obtained, and will be used in the determination of the structure of AQP-1 by multiple isomorphous replacement. Models derived from electron crystallography of two-dimensional crystals will also be used to assist in phase determination. The structure at 4A resolution will define the architecture of the aqueous pore and describe the protein fold. Next, crystals of AQP-I will be improved for the determination of a complete structure at higher resolution. Lastly, recombinant aquaporins other than AQP-1 will be produced for crystallization experiments. The ultimate goal is the determination of an ensemble of human aquaporin structures for use in structure- based drug design.